Rotary drum dryers for drying bulk solids are well-known in the art and generally consist of a horizontal drum which is rotated about its horizontal axis and associated with a heating source for drying material loaded into the drum. Drum dryers are typically heated by firing a burner along the axis of the drum or by directing combustion gas into the drum through one end. In many instances it is desirable to control the flow of gases into and out of the dryer drum. In particular, when drying combustible materials, it is often desirable to control the oxygen content in the drum to prevent combustion of the material being dried.
Rotary dryer drums are used, for example, in the wood processing industry, to dry hogfuel and other wood wastes for boiler fuel or to dry wood strands for strand board production. Such dryer drums are typically sealed to a combustion gas inlet plenum and an exhaust gas outlet plenum. The gases which are supplied by the inlet plenum are typically combustion gases depleted in oxygen. The interior of the drum typically runs at a negative pressure produced by the natural or forced draft of the exhaust gas chimney. Existing dryers utilize resilient, flexible seals which are mounted on the non-moving parts of the dryer and ride on a sealing ring mounted on the rotating drum. These existing drum dryer seals have been typically manufactured of built-up layers of fiberglass or ceramic cloth treated with silicone rubber. While these seals function quite satisfactorily when the dryer is initially introduced into service, over time problems can rise.
The prior art seals are initially resilient and are installed so that they are biased by their resiliency against the sealing ring. The seals, while normally running relatively cool, will on occasion be subject to temperatures as high as 1500 degrees Fahrenheit. Because the drum is normally under negative pressure, hot combustion gases do not normally impinge upon the seals. However, on occasion, the drum outlet can become clogged and combustion gases will back up through the seals subjecting them to high temperatures.
In practice, these conditions have meant that after a few months, the prior art seals loose some of their resiliency and thus are no longer effective at sealing outside air from the interior of the rotating drum. The problem of obtaining a good seal between the dryer and the inlet plenum is further aggravated by the tendency of rotating drum dryers to be slightly eccentric about their axes of rotation. This eccentricity tends to increase with age of the dryer as wear of the dryer trunnions and other components tends to increase the eccentricity of the dryer drum.
Another factor which tends to make sealing the rotating drum difficult is the changes in size of the drum with changes in temperature, as the drum upon heating tends to increase in diameter and length. What is needed is a rotary drum dryer seal which remains resilient and functional after prolonged exposure to high temperatures.